All depends on genes if you ask me. If the parents are nice as adults then it's very likely the pups will as well.
Absolutely Marbles can loose thier pattern/beauty. I have first hand experience with a pup that was absolutely amazing as a pup. By 12" she was average at best. This is why people don't understand the pricing. If you know something's gonna be nice, and stay nice it's worth more then a gamble.
Big nice Marbles ARE uber rare. A vast majority will loose thier patterns with age. Even the nice ones may deteriate, just not as much.
Enviroment plays a huge factor. The color of substrate, background, especially lighting will all effect them differently. Water quality is another factor. I've seen great improvement in several of mine after they went into the 750. The filtration is rediculous. The tank they came from even had halide lighting on it oposed to the 750 having flourescents.
I'm a big fan of variables. Testing everything. Try something, note the change, try it again. What works for some may not for others but if it seems to repeat there's usually some truth to it. I've spent the last 3+ years trying to crack Marbles and it took until just lately to feel as though I'm even starting to understand things and that's came from experience, just paying attention to mine and others as they grow and develope. We won't find out anything rock solid until we're several generations deep, but it's pretty easy to draw conclusions from what we've seen so far.